Ba1-xSrxTiO3 (BST) in thin film form is an important material for utilization in tunable microwave devices, such as filters and phase shifters. For these tunable microwave devices, high dielectric tunability, low microwave loss, and good temperature stability are required for optimum performance and long-term reliability. The current generation of tunable microwave phase shifter devices is based on single composition, paraelectric Ba1-xSrxTiO3 films, and the military and commercial end-users have expressed significant concern that in practical applications, e.g., On-The-Move (OTM) phased array antennas, the phase shifter performance will be compromised due to the temperature dependence of the device capacitance. Specifically, the capacitance of the BST based device is strongly influenced by temperature changes because the dielectric constant (∈r) of a single composition paraelectric BST films (e.g. Ba0.5Sr0.5TiO3) follows the Curie-Weiss law;K=Ccurie/(T−θ)  (1)where K is the dielectric constant Ccurie is the curie constant, T is the temperature, and θ is the Curie temperature. Spurious changes in the device capacitance that stem from ambient temperature fluctuations will disrupt the phase shifter performance via device-to-device phase shift and/or insertion loss variations leading to beam pointing errors and ultimately communication disruption and/or failure in the ability to receive and transmit the information. The same is true for BST based tunable filters where the capacitance susceptibility to temperature changes results in the alteration of the band pass window sharpness (window narrows or broadens), or the entire band pass window may shift to higher or lower frequencies and/or the insertion loss may be degraded. Such poor temperature stability of the capacitance would result in the carrier signal drifting in and out of resonance on hot and cold days.
Traditional approaches to address the issue of device (phase shifter and/or tunable filter) temperature instability have focused on employing hermetic or robust packaging, whereby the robust package serves to protect the tunable device from the harsh environmental extremes. Although this approach is successful, hermetic/robust packaging would add significant cost, size, and weight to these OTM phased array antennas, which in turn violates the military and commercial sectors requirements for affordability. It is not foreseeable that such an approach could meet the criteria of a low cost phase shifter i.e. ˜$5.00 per phase shifter element. Other concepts to achieve temperature stability compliance, involve the use of “system heat sinks” and/or cooling apparatuses such as “mini fans” and/or “temperature compensation circuits” or “mini ovens-heating units”. Such thermal management solutions (fans/heat sinks/ovens and various other types of thermal management) may be utilized with the OTM antennas: however they will add extra weight, size, and cost to the overall system, and as such, are deemed unacceptable. Temperature compensation can also be achieved using either the (1) curve fit or (2) look up table approach. The curve fit approach centers on the formulation of a temperature dependant mathematical expression/equation, which represents the drift of each BST tunable device. A microprocessor utilizes this equation and the ambient temperature data (obtained from a thermocouple mounted on the printed circuit board) to calculate the tuning voltage. The look-up table approach, as its name implies involves using a look up table. In order to obtain the look-up table coefficients, the phase shifter characteristics must be measured at discrete temperatures then the BST bias voltage is manually adjusted to maintain the phase shifter specifications. In the worst-case scenario, one would have to obtain a set of points for each temperature (i.e., 23° C., 24° C. etc.). Typically one would expect to have a small subset of temperature/bias points for each bias line. The exact number of points is of course dependent on the BST devices, the other phase shifter components, and the phase shifter topology. Unfortunately, this approach can be quite complex as there usually isn't a one size fits all solution. The calibrations are also labor and time intensive and are useful if only a limited number of OTM phased array antennas are to be fielded. Common-place materials science approaches for reducing the temperature dependence of an active material have been to select the temperature interval of operation well above the temperature corresponding to the active materials permittivity maximum. Unfortunately, this method results in reduced material tunability and the temperature coefficient of capacitance (TCC) is still too high for practical military applications such as OTM phased array antennas.
Thus the development of a temperature stable phase shifter/filter technology capability is paramount, as it ensures uninterrupted reliable information exchange via On-The-Move communications systems in harsh temperature environments.
Thus, there is a continuing need for a temperature stable material that possesses low dielectric loss and high tunability, particularly for use in phase shifting and/or tunable preselector (filter) devices for communications systems, as well as methods for fabrication of such temperature stable enhanced property materials.